A conventional valve opening and closing control device for regulating valve timing of an engine using a rotational torque of a motor is disclosed in, for example, JP-UM-A-4-105906 (hereafter “Patent Document 1”). Also, a valve opening and closing control device that regulates a maximum valve lift in an engine making use of a rotational torque of a motor is disclosed in, for example, JP-A-11-324625 (hereafter “Patent Document 2”).
These control devices require a small sized motor to be able to be mounted on an engine and that the motor be supplied with an electric current of, for example, 20 A or more, preferably, 40 A or more to generate a large rotational torque. A motor drive device 1 for such a control device is shown in FIGS. 15A–15B.
The control device 1 drives a three-phase motor and is provided with a bridge circuit 2 with a motor as a load. The bridge circuit 2 comprises three rows of arms that comprise two switching elements 3a, 3b connected in series to each other and two diodes 4a, 4b connected in parallel to the corresponding switching elements 3a, 3b. The respective arms 5u, 5v, 5w are connected mutually in parallel to an electric source, and respective non-connected terminals of windings 8u, 8v, 8w of the motor star-connected together are connected to points 7u, 7v, 7w of interconnection of the two switching elements. 3a, 3b on the respective arms 5u, 5v, 5w. A control circuit (not shown) connected to gates of the respective switching elements 3a, 3b controls ON/OFF of the respective switching elements 3a, 3b to carry an electric current to the windings 8u, 8v, 8w of the motor. For example, in order to carry an electric current to the windings 8u, 8v as indicated by alternate long and two short dashes lines with arrows in FIG. 15A, the switching element 3a on an upper stage side of the arm 5u connected to the winding 8u and the switching element 3b on a lower stage side of the arm 5v connected to the winding 8v are put in an ON state.
In the motor drive device 1, when, for example, one 3a of the two switching elements 3a, 3b of arm 5V is put in an ON state as shown in FIG. 15B in order to carry an electric current to the windings 8u, 8v, a circulating electric current as indicated by alternate long and two short dashes lines with arrows in FIG. 15B flows through a path that connects the diode 4a connected in parallel to a separate switching element 3a on the same arm 5v as that, on which the switching element 3b is disposed, the switching element 3a on the arm 5u in an ON state, and the windings 8u, 8v. When the circulating electric current flows, residual voltage between the nonconnected terminals of the windings 8u, 8v decreases.
Since voltage drop of a diode is generally in the order of 0.7 V, a large heat loss as large as 28 W is caused when a circulating electric current of, for example, 40 A flows through the diode. In the motor drive device 1 of a valve opening and closing control device of a type making use of a motor, frequency of ON/OFF switching of the switching elements 3a, 3b is high, so that the circulating electric current causes the diodes 4a, 4b to generate much heat. Excessive heat generated by the diodes 4a, 4b incurs failure in constituent elements, such as the switching elements 3a, 3b, etc. of the motor drive device 1. Hereupon, when frequency of ON/OFF switching of the switching elements 3a, 3b is made low in order to restrict heat generated by the diodes 4a, 4b, the driving performance of the motor and hence the valve opening and closing control performance is degraded.